1. Field of the Invention
This invention relates to a method for producing dried particulate coal fuel and electricity from a low rank particulate fuel by drying the particulate low rank coal in a coal drying zone using a heated combustion gas from a coal combustion zone associated with the production of electricity to dry the low rank particulate coal in the coal drying zone and to pre-heat the boiler combustion air by first using the air to cool the dried coal.
2. Background
In many instances, coal as mined contains undesirably high quantities of water for transportation and use as a fuel. This problem is common to all coals, although in higher rank coals such as anthracite and bituminous coals the problem is less severe because the water content of the coal is normally lower and the heating value of such coals is higher.
The situation is different with respect to lower rank coals such as sub-bituminous, lignite and brown coals. Such coals as produced typically contain from about 15 to about 65 weight percent water. While many such coals are desirable as fuels because of their relatively low mining cost and since many such coals have a relatively low sulphur content, the use of such lower rank coals as fuel has been greatly inhibited by the fact that as produced they typically contain a relatively high percent of water. Attempts to dry such coals for use as a fuel have been inhibited by the tendency of such coals, after drying, to undergo spontaneous ignition and combustion in storage, transit or the like. The drying required with such low rank coal is a deep drying process for the removal of surface water plus the large quantities of inherent (interstitial) water present in such low rank coals. By contrast, when higher grade coals are dried, the drying is commonly for the purpose of drying the surface water from the coal particle surfaces but not inherent water since the inherent water content of the higher rank coals is relatively low. As a result, short residence times in the drying zone are normally used and the interior portions of the coal particles are not heated since such is not necessary for surface drying. Typically the coal leaving the dryer in such surface water drying processes is at a temperature below about 110.degree. F. (45.degree. C).
By contrast, processes for the removal of inherent water require longer residence times and result in heating interior portions of the coal particle. The coal leaving a drying process for the removal of inherent water will typically be at a temperature of about 130.degree. to about 250.degree. F. (54.degree. to 121.degree. C.). When such processes for the removal of inherent water are applied to low rank coals, the resulting dried coal has a strong tendency to spontaneously ignite, especially at the high discharge temperatures, upon storage, during transportation and the like.
The tendency to spontaneously ignite can be reduced by cooling the dried low rank coal to a temperature below 100.degree. F. (38.degree. C.) and preferably below about 80.degree. F. (27.degree. C.) promptly after the drying process. The tendency to spontaneously ignite can be further reduced by use of deactivating fluids. Various processes for the drying, coating and deactivation of low rank coals are shown in U.S. Pat. Nos. 4,396,394; 4,396,395; 4,401,436; 4,402,706; 4,402,707; 4,421,520; 4,495,710; 4,498,905; 4,501,551; 4,527,342; 4,545,132 and 4,571,174. These patents are hereby incorporated in their entirety by reference.
While drying low rank coals as disclosed in these patents results in the production of a dried low rank coal having a reduced tendency to spontaneously ignite, such processes require the generation of a heated drying gas at a mine site or other remote location. The drying of the low rank particulate coal is best accomplished at a mine site or near a mine site to obtain the advantages of reduced transportation costs resulting from the reduction in the weight of the low rank coal fuel. Combustion processes of the types required to generate the drying gas are not normally conducted in such areas and when conducted require the installation of combustion equipment and scrubbing equipment for clean-up treatment of the exhaust gases to remove sulfur oxides, particulates and the like.
Such processes also require a fan system for controlled flow of inlet air and a fan for controlled flow of exhaust gases.
Further, the use of such coal drying processes results in the production of a large portion of the dried coal fuel as fines, i.e. coal particles of a size consist smaller than about 80 Tyler Mesh. While such finely divided coal is highly desirable for use as a fuel it is difficult to transport and frequently constitutes a waste at the coal drying site. As a result while such coal drying processes result in the production of a dried low rank coal fuel having an increased BTU content which can be transported more economically, the processes require the installation of combustion equipment, fan equipment and exhaust gas clean-up equipment at remote sites and result in the loss of a portion of the low rank coal charged to the drying process to waste.
Electric generating facilities are frequently coal fired. The use of coal fired furnaces of a variety of types such as tangentially fired dry bottom furnaces, fluidized bed furnaces and the like for the generation of electricity is well known and such furnaces require that the coal fed to the furnace be finely divided. The finely divided coal is combusted to produce a hot combustion gas which is passed through a series of heat exchange zones to produce heated and superheated steam for use in driving turbines to produce electricity. While the use of finely divided coal is required in such coal fired furnaces the difficulty in transporting the finely divided coal prohibits transportation of the finely divided coal produced in the drying process for long distances for this purpose since excessive amounts of the finely divided coal are lost during transportation and constitute not only a lost product but an undesirable pollutant. Accordingly larger particles of dried low rank coal are transported for grinding to finely divided particles prior to charging the dried low rank coal to the furnace. Generally coal fired furnaces require the use of exhaust gas clean-up treatment facilities to remove sulfur oxides and particulates from the exhaust gas prior to discharging the exhaust gas to the atmosphere.
According to the present invention a synergistic improvement in the use of low rank coal for use as a fuel to generate electricity is accomplished by producing dried particulate low rank coal fuel and electricity from a low rank particulate coal by a method consisting essentially of:
(a) charging particulate low rank coal to a coal drying zone wherein the coal is supported on a first support means, the first support means being adapted to the flow of a hot gas upwardly through the support means and the particulate low rank coal;
(b) drying the particulate low rank coal in the coal drying zone by flowing a hot gas upwardly through the first support means and the particulate low rank coal to produce the dried particulate coal fuel;
(c) charging a portion of such dried particulate coal to a coal burning unit for combustion to produce a hot combustion gas stream;
(d) charging a portion of the hot combustion gas to a heat exchange zone to produce steam;
(e) charging at least a portion of the steam to an electrical generator to produce electricity; and
(f) charging a portion of the hot combustion gas to the coal drying zone as hot gas.